The present invention resides generally in the field of chlorinated pyridines. More particularly, the present invention relates in one preferred aspect to processes for preparing 2,3,5-trichloropyridine by dechlorinating 2,3,5,6-tetrachloropyridine.
As further background, polychlorinated pyridine derivatives are important intermediates in the preparation of pesticides. Consequently, much effort has been made at both the academic and industrial levels to find improved, economically-practicable processes for their preparation.
2,3,5-Trichloropyridine is one such intermediate. Proposed processes for its production have been widely diverse and have included both the selective chlorination of intermediate compounds, the selective dechlorination of higher-chlorinated pyridines, and other routes.
For example, 2,3,5-trichloropyridine can be prepared by reacting pyridine and phosphorous pentachloride as described by Sell et al., C. J. Chem. Soc. 73, 437 (1888). It can also be prepared by chlorinating pyridine hydrochloride with chlorine gas at 115.degree. C.-120.degree. C. as described by Sell, J. Chem. Soc. 93, 437 (1908). 2,3,5-Trichloropyridine can also be prepared by treating pyridine hydrochloride with liquid chlorine at 80.degree. C. to 225.degree. C. at an HCl pressure above 30 psig as disclosed in U.S. Pat. No. 3,732,230, or by the reaction of N-methyl-3,5-dichloro-2-pyridone with phosgene as taught in Ann. Chem. 486,71,78 (1931).
U.S. Pat. No. 4,108,856 discloses producing 2,3,5-trichloropyridine by reacting 3,5-dichloropyridine with chlorine at an elevated temperature in the range of 300.degree. C. to 460.degree. C. in the presence of a diluent such as chloroform or carbon tetrachloride. U.S. Pat. No. 4,287,347 discloses the production of 2,3,5-trichloropyridine by reacting 3,5-dichloro-2-pyridone with phosgene in the presence of certain N,N-disubstituted formamides and an inert solvent. 2,3,5-Trichloropyridine can also be produced by a liquid phase chlorination of 3,5-dichloro-2-trichloromethylpyridine at a temperature of 170.degree. C. to 220.degree. C. as taught in U.S. Pat. No. 4,563,531.
U.S. Pat. No. 4,111,938 reports that 2,3,5-trichloropyridine can be produced by reacting 2,3,5,6-tetrachloropyridine (Symtet) using zinc dust in a heterogeneous medium containing an alkaline reagent. The typical conditions of this '938 patent involve heating a mixture of about 1 equivalent of Symtet, 2 equivalents of zinc dust, 8 equivalents of aqueous sodium hydroxide (8N) and toluene at reflux for about 7 ours. U.S. Pat. No. 4,127,575 discloses a conversion of 2-hydrazino-3,5,6-trichloropyridine to 2,3,5-trichloropyridine using NaOCl as an oxidizing agent.
Symtet has also been converted to 2,3,5-trichloropyridine by hydrogenolysis using palladium on carbon as catalyst, as reported in JP 63275565. Reported conversion was very low, as was yield. U.S. Pat. No. 4,258,194 reports that 2,3,4,5-tetrachloropyridine can be converted to 2,3,5-trichloropyridine with zinc dust and ammonium salts of methane phosphonic acid monomethylester in methane phosphonic acid dimethylester. This method, however, failed to convert Symtet to 2,3,5-trichloropyridine.
Other routes to 2,3,5-trichloropyridine have also been reported, including the direct chlorination of 2-chloropyridine (JP 01100158) or 2,6-dichloropyridine (JP 58206564), and ring synthesis (EP 12117).
Despite the above-reported work, there remain needs for improved processes for the practicable, commercial-scale production of 2,3,5-trichloropyridine and other similar polychlorinated pyridines. The present invention is addressed to these needs.